Doctors could take industrial action over the government's "unwarranted and unfair assault" on their pensions, the profession's leader has warned ministers.

Dr Hamish Meldrum, chairman of the British Medical Association, delivered a sharply worded rejection of the coalition's pensions plans in his keynote address to the doctors' union's annual conference on Monday.

"Let me make it absolutely clear: we will consider every possible, every legitimate action that can be taken to defend doctors' pensions," he said in a speech to 500 doctors' representatives in Cardiff.

"I have this message for ministers. Whilst we will be reasonable, whilst we will not rush to precipitate action, whilst we will not put patients' lives at risk, do not in any way or for one single moment mistake this responsible attitude as a reason to underestimate our strength of feeling and our determination to seek fairness for those we represent.

"The profession will act responsibly, but we will not accept an unwarranted and unfair assault on our pensions."

Meldrum's remarks, which were greeted with loud applause, underline the strength of feeling among doctors over changes which he said should not be "just a poorly concealed tax on public sector workers".

A doctors' strike is highly unlikely. But there are other forms of industrial action they could take in order to apply pressure, although doctors have not done so since a work-to-rule over contracts in 1975.

The BMA leader was speaking ahead of a pensions debate by delegates on Thursday, where there will be discussions of what steps to take if ministers ignore their concerns.

There are calls for the union "to ballot the BMA membership regarding all forms of industrial action" if there is no rethink.

Dozens of conference motions on the issue show that doctors feel strongly about the possibility of paying more in contributions and losing their final salary scheme.

Meldrum called the government's pensions plans "another major threat hanging over the profession".

He also criticised Danny Alexander, the chief secretary to the Treasury, for, he said, making the job of negotiating a settlement "impossible, if all we hear are public ultimatums and ridiculous threats such as those we heard just a few days ago".

Meldrum said that big changes to the NHS pension scheme were unnecessary as it was updated as recently as 2008.

"Fact: even before the imposed change from RPI to CPI and the threat of the imposed Treasury levy, the NHS superannuation scheme was in surplus, and by 2015 the scheme will have contributed over £10bn to the Treasury," he said.

The post-2008 scheme already has tiered contributions to guarantee fairness for lower-paid workers, while all members of the scheme are paying more into it, with the highest-paid contributing 8.5% of their salary, he added.

Dr Andrew Dearden, chairman of the BMA's pensions committee, has warned that if ministers press ahead with their plans more doctors could opt for early retirement in order to benefit from the more advantageous current system.

A doubling of the usual rate of retirements could lead to contributions going down at the same time as costs rise, he said.

Meldrum also warned that, despite the recent changes made to the health and social care bill, the BMA would scrutinise the government's 180 amendments to ensure that "what's been promised is being delivered [and] that this isn't just a case of turning a pig's breakfast into a dog's dinner".

He also urged NHS bosses not to "slash and burn" as they try to operate within the tightest financial climate in the service's 63-year history.

And, while stressing that doctors are not against competition, he reiterated the BMA's opposition to attempts to introduce a free market into the NHS.